Green synthesis of SiO2 nanoparticles from Rhus coriaria L. extract: Comparison with chemically synthesized SiO2 nanoparticles

The usage of the green synthesis method to produce nanoparticles (NPs) has received great acceptance among the scientific community in recent years. This, perhaps, is owing to its eco-friendliness and the utilization of non-toxic materials during the synthesizing process. The green synthesis approach also supplies a reducing and a capping agent, which increases the stability of the NPs through the available phytochemicals in the plant extractions. The present study describes a green synthesis method to produce nano-silica (SiO2) NPs utilizing Rhus coriaria L. extract and sodium metasilicate (Na2SiO3.5H2O) under reflux conditions. Sodium hydroxide (NaOH) is added to the mixture to control the pH of the solution. Then, the obtained NPs have been compared with the chemically synthesized SiO2 NPs. The structure, thermal, and morphological properties of the SiO2 NPs, both green synthesized and chemically synthesized, were characterized using Fourier-transform infrared spectroscopy (FTIR), Ultraviolet-Visible Spectroscopy (UV-Vis), X-ray diffraction (XRD), and Field Emission Scanning Electron Microscopy (FESEM). Also, the elemental compassion distribution was studied by energy-dispersive X-ray spectroscopy (EDX). In addition, the zeta potential, dynamic light scatter (DLS), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC) was used to study the stability, thermal properties, and surface area of the SiO2 NPs. The overall results revealed that the green synthesis of SiO2 NPs outperforms chemically synthesized SiO2 NPs. This is expected since the green synthesis method provides higher stability, enhanced thermal properties, and a high surface area through the available phytochemicals in the Rhus coriaria L. extract.


Introduction
Researchers have recently focused on nanotechnology due to the extraordinary growth and development in this field, with the expectation that it will improve the current standard of In this study, a certain amount of nano-silica was produced by the green method using Rhus coriaria L. extract. At the same time, a certain amount of nano-silica was chemically synthesized using Sol-gel method [44]. For both samples, numerous characterization techniques, including Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), Zeta potential, field emission scanning electron microscopy (FESEM), Transmission Electron Microscope analysis (TEM), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) analysis, and dynamic light scattering (DLS) were utilized. Consequently, it was found that the green synthesized silica NPs, in general, possess superior properties over the chemically synthesized silica NPs.

Materials and methods
Sodium metasilicate (Na 2 SiO 3 .5H 2 O) Molecular weight 212.14 g/mol, and purity >98.8 percent was used for synthesizing nanosilica using the green synthesis method. Sodium hydroxide (NaOH) with a Molecular weight of 40 g/mol was used to control the pH value of the growth solution. A sealed bottle of commercial nanosilica with 99% purity has been used in this study for comparison. All these materials and salts were ordered from Sigma-Aldrich Company. In addition, Rhus coriaria L. seeds were provided from Akre city, located in the Dohuk Governorate in the Kurdistan Region of Iraq. The latitude and longitude coordinates of Akre city are 36.741390 and 43.893333. The hot plate temperature was set at 80˚C during the SiO 2 nanoparticles synthesis. The chemically synthesized SiO 2 NPs were prepared using Verma and Bhattacharya's procedure [44].

Characterization
To study the structure of SiO 2 nanoparticles, X-ray diffraction (XRD) machine was utilized by using PANanalytical (Cu K = 1.54 nm). The scanning value was 1˚per minute in the scope of 10˚up to 80˚. To confirm the formation of SiO 2 NPs, the UV-Vis technique was performed using a double beam spectrophotometer (Super Aquarius Spectrophotometer-1000). Field emission scanning electron microscopy (FESEM Quanta 450) was used to study the morphology and grain dispersion. The elemental composition of the produced nanostructures was determined using dispersive X-ray spectroscopy (EDX), installed inside FESEM. The functional groups from leaf extracts attached to the SiO 2 NPs were studied by attached to the SiO 2 NPs using a Fourier transform infrared (Perkin Elmer FTIR) spectrophotometer in the acquisition range of 400-4000 cm −1 . Dynamic light scattering was used to determine the size of SiO 2 NPs. The DSC curve was performed using a differential scanning calorimetry (DSC) type (TA Instruments USA, DSC Q10) in the temperature range of 50-600˚C. Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer-Pyris1 analyzer (USA).

Preparing of Rhus coriaria L. extract
Rhus coriaria L. seeds were collected and dried in a shadow area at 25˚C, then crushed using a grinder machine. Then, 50 g of crushed Rhus coriaria L. was heated by magnetic stirring in 100 ml of double-distilled water for 40 minutes. The final mixture was filtered through the filter paper, and the pure extract was stored in the refrigerator for further use.

Silica nanoparticles (SiO 2 NPs) synthesis using the green method
20 mL Na 2 SiO 3 was added dropwise to 100 mL of freshly generated Rhus coriaria L. extract while the mixture was being stirred at 60˚C. The procedure was carried out under reflux conditions for 12 hours at pH 9 until the SiO 2 NPs were synthesized. The pH value of the solution was adjusted using 0.1 M NaOH, which was dissolved well in 50 mL of double-distilled water. Therefore, the mixture was filtered, and the obtained precipitate was washed carefully with double distilled water three times and methanol to remove all contamination. Centrifugation at 7000 rpm for 25 minutes separated the residues from the mixture, then heated at 550˚C for 45 minutes in a furnace to eliminate any contaminants and organic elements around the SiO 2 NPs. Then as indicated in Fig 3, the residue was dried in an oven and kept for further characterization and application activities.

Characterization of green synthesized (GS) and chemically synthesized (CS) SiO 2 NPs
The morphology, structure, optical, toughness, and thermal properties of both types of SiO 2 NPs have been investigated using several characterization techniques.

FTIR Spectroscopy.
Infrared spectroscopy has been extensively used to identify the functional group's modification and bonding properties of nanomaterials. The FTIR spectrum analysis might give valuable information about the structure, chemical composition, and purity of NPs. It can also identify and classify probable biomolecules that can be reliable for capping, leading to proficient stabilization of the NPs. The samples containing the NPs are entirely dried and ground with KBr pellets and investigated. Scanning is confirmed to acquire a covered signal-to-noise ratio for suitable consequences.  [4,45]. While from Fig 4B, it is clear that the bands nearby 3468 cm -1 is associated with the Si-OH, which is usually characteristic of the OH group corresponding to the available phytochemical in Rhus coriaria L. extract [37]. In addition, the band at 1637 cm -1 is caused by the scissor bending vibration of water molecular H 2 O. Similarly, the peaks at 1097 and 802 cm -1 correspond to the asymmetric stretching of the Si-O-Si and Si-O band bending vibrations, respectively. Moreover, the band at 463 cm -1 was connected with the bending vibration of O-Si-O modes [46].

X-ray diffraction.
To study the crystalline structure of both chemical approach and green synthesized of SiO 2 NPs, the X-ray diffraction (XRD) analysis was carried out as shown  in Fig 5. For both, the commercial SiO 2 NPs (Fig 5A) and the green synthesized SiO 2 NPs (Fig  5B), a broad peak centered at 2θ = 23˚(hkl = 100) is observed, which can be assigned to the SiO 2 NPs and no other diffraction peaks can be detected [4].  [47]. Also, in both cases, the wide peaks display a whole amorphous structure. Nevertheless, the green synthesized SiO 2 NPs show relatively lower noise than the chemically synthesized SiO 2 NPs. It can be stated that the peaks were matched with the reference JCPDS file No. 89-0510 for SiO 2, and it discloses no impurities peak for SiO 2 [48].
The particle size (D) of the CS-SiO 2 NPs and GS-SiO 2 NPs was calculated using Debye-Scherer equation [49].
Where λ is the wavelength of the utilized X-ray (λ = 1.5406 Å), β is the full width at half maximum (FWHM) of the most intense peak, and θ is the Braggs angle. From the previous equation, the SiO 2 NPs sizes were 18 nm and 23 nm for the CS-SiO 2 NPs and the GS-SiO 2 NPs, respectively. The absence of sharp peaks shows that there is no crystalline structure of the SiO 2 NPs. The GS-SiO 2 NPs produced by Rhus coriaria L. extract is an outstanding process, and the yield is close to 100 percent. This is a very encouraging result, and it was expected since plant extracts were utilized instead of the toxic chemical. Also, the phytochemicals present in the plant extracts acted as a reduction, capping, and stabilizing agent of the GS-SiO 2 NPs.

FESEM analysis.
The surface morphology of the CS-SiO 2 NPs and the GS-SiO 2 NPs was primarily determined by FESEM, and the results are shown in Fig 6. It can be seen that the majority of the SiO 2 NPs in both samples are on the nanoscale and possess a spherical shape. The main difference between the CS-SiO 2 NPs and the GS-SiO 2 NPs is that the green synthesized method produces polydisperse NPs, while in the chemically synthesized sample, most of the NPs are monodisperse NPs with different particle sizes. The formation of tiny particles as a result of a rise in the concentration of the OH group causes a decrease in the rate of condensation and hydrolysis process, which in turn inhibits the formation of bigger particles. Furthermore, the SiO 2 nanoparticles are somewhat agglomerated, which is characteristic of the green synthesis method. This is attributed to the fact that nanoparticles have a larger surface area and long-lasting affinity throughout the dehydrating process, which causes agglomeration of the particles [22,23]. It can be noted that the phytochemicals inside plant extraction have a significant effect on the stability of NPs and their agglomeration. As a result, the SiO 2 NPs adhere to one another and spontaneously produce asymmetrical clusters [24,26]. The process for fabricating the SiO 2 NPs is related to a number of growth factors, including the growth time, the concentration of plant extract, the pH value of the solution, the temperature of the solution, and the concentration of salt. Consequently, calibration of these growth components is critical to accomplishing the desired shape and size of nanoparticles to allow for their maximal modification and request.   synthesized NPs. This outcome agrees in high precision with the DLS results discussed in the next section. The results of this study agree with the results obtained by Yadav and Fulekar [50].

Dynamic light scattering (DLS).
DLS is used to calculate the consistent size of the SiO 2 NPs distributed in the water medium. The size distribution profile of the chemically and green synthesized SiO 2 NPs using the DLS technique is shown in Fig 8. This study found that the average diameter for chemically and greenly synthesized SiO 2 NPs was 32 nm and 60 nm. DLS measurements (Fig 8) showed that the hydrodynamic diameter of the SiO 2 NPs stayed nearly unchanged during the measurement for one hour. The green synthesis method generally provides smaller NPs than the chemical methods [46].
Additionally, the CS-SiO 2 NPs remained monodisperse throughout the DLS test. The statement that some engineered NPs, such as SiO 2 , tend to combine and agglomerate speedily to form bigger particles, with sizes beyond the nanoscale, might bring together an extra level of difficulty to describe particle performance in the aqueous medium [51]. Nevertheless, this larger agglomeration will normally resolve swiftly by using an ultrasonic bath. Several researchers have achieved comparable results [52].
3.1.6. Zeta potential. Zeta potential is one of the most significant factors that explain the rheological performance of ceramic suspensions. The zeta potential is a measurement of the real electric field around the surface of the nanoparticle [53]. When a nanoparticle is charged totally, the amount of the charge is calculated by the ion concentration versus different charges close to the surface of the NPs. The greater the level of zeta potential, the stronger the surface repulsion/attraction. For the determination of the Zeta potential, it is necessary to consider the mobility of nanoparticles under the influence of an applied electromagnetic field. This work demonstrated that the capping molecules present on the surface of the green produced SiO 2 NPs are mostly made of negative charge families and that they are also accountable for the stability of the NPs in the growing solution, as shown before [54].
The Zeta potential gives information about the stability of the particles. Fig 9 shows the zeta potential analysis for both the CS-SiO 2 NPs and the GS-SiO 2 NPs. It can be regarded that the values of zeta potential for CS-SiO 2 NPs and the GS-SiO 2 NPs are -38 mV and -45 mV, respectively. This is a good indicator that GS-SiO 2 NPs provide stable NPs. The variation of the zeta potential value between the CS-SiO 2 NPs and the GS-SiO 2 NPs is most likely due to the agloromation that has been shown in the SEM analysis. To a great extent, agglomeration is associated with the green synthesis method. This is more likely due to the insufficient number of capping agents around the NPs in some plant extracts [55]. Conventional methods for synthesizing NPs involve high-priced chemical and physical procedures that frequently utilize hazardous ingredients, most of which are carcinogenic. These issues arise mostly from utilizing organic solvents that work as reducing, capping, and stabilizing agents that prevent the NPs from agglomerating. Consequently, the presence of these toxic creation agents during the synthesizing process limits the application of the NPs in environmental and biomedical applications [56]. Accordingly at present, researchers are focusing on a harmless, consistent, inexpensive, and environmentally-friendly method to prepare NPs.
3.1.7. Differential scanning calorimetry (DSC) analysis. The morphology analysis of the SEM images shows that the GS-SiO 2 NPs possess agglomeration. More importantly, the agglomeration can be affected by the temperature. Thus, the thermal behavior of SiO 2 NPs is investigated by DSC analysis at temperatures varying from 25 to 1000˚C as shown in Fig 10. The slow heating rate of 10˚C/min was chosen to observe similar behavior for both chemically and green-synthesized SiO 2 NPs. From Fig 10, it can be regarded that the DSC curve of CS-SiO 2 NPs clearly shows one endothermic peak, centered at 115˚C, and one exothermic peak, centered at 537˚C. While the DSC curve of GS-SiO 2 NPs shows one endothermic peak, centered at 190˚C, and two exothermic peaks, centered at 55˚C and 522˚C, respectively. The endothermic peak temperatures of 115˚C and 190˚C, which were began at 100˚C, for both the CS-SiO 2 and the GS-SiO 2 NPs may be due to dehydration, which is the simultaneous removal  of physisorbed surface moisture of SiO 2 NPs. This is a very good indicator that the GS-SiO 2 NPs are stronger absorbers of the surrounding moisture. This obvious endothermic peak shift is more likely due to the dihydroxylation due to the hydroxyl groups (OH) condensation in GS-SiO 2 NPs. Kim et al. [57] stated that the right reduction of SiO 2 NPs occurs at temperatures ranging from 150˚C to 550˚C. As a result, it can be decided that this substantial weight reduction is related to removing water and the decomposition of some of the functionalizing agents [58]. This decomposition, accompanied by an exothermic peak, occurs between 400 and 550˚C. Similar results have been found by other authors [59], stating that above 600˚C, the inaccessible hydroxyl groups are gradually eliminated, resulting in a little mass loss. Another significant trend in the DSC curve is that the GS-SiO 2 NPs possess more heat capacity than the CS-SiO 2 NPs. This can be reflected directly on the heat flow rate in both cases. Again, this is an excellent indicator that the GS-SiO 2 NPs are better than the CS-SiO 2 NPs, especially in solar cells, electronic devices, and engineering applications [60]. However, DSC determines the amount of energy absorbed or released by a sample throughout the cooling or heating procedure. The thermal stability of both CS-SiO 2 and GS-SiO 2 NPs as a function of temperature has been investigated using the TGA technique. Fig 11 shows the TGA state of both CS-SiO 2 and GS-SiO 2 NPs. Through comparing their TGA curves, it can be seen that the GS-SiO 2 NPs are more thermally stable than the CS-SiO 2 NPs. It loses only 7% of its weight at~400˚C. The CS-SiO 2 NPs lose~90% of their weight at this temperature. This is no surprise since the DSC analysis reinforces these consequences. This remarkable weight loss in the CS-SiO 2 NPs could be attributed to the decomposition of the attached organic groups [61]. On the other hand, Fig 11 indicates that both CS-SiO 2 and GS-SiO 2 NPs lose about 4% of their weight due to water molecules' evaporation. The significant weight loss occurred between 200-450˚C, and 200-750˚C for CS-SiO 2 and GS-SiO 2 NPs, respectively. Again, this is a very good indicator that the capping and stabilizing agents around the GS-SiO 2 NPs are strong enough to resist high temperatures [62]. While the major weight loss for CS-SiO 2 NPs occurs at a temperature~400˚C, which is~92%. Consequently, this study showed that GS-SiO 2 NPs could be utilized in high-temperature devices and applications [60].

Conclusions
This study demonstrated that spherical, nano-sized, thermally stable SiO 2 NPs could be synthesized using a green method. This method is eco-friendly, one-pot, cheap, productive, and leads traditional synthesizing methods. Rhus coriaria extract, as a reducing, capping, and stabilizing agent, used with sodium metasilicate (Na 2 SiO 3 .5H 2 O), as a precursor, under reflux conditions and pH 9. Several The shape, purity, crystal structure, stability, thermal, and optical characteristics of biosynthesized SiO 2 were investigated using characterization methods. Considering all of these aspects, the green synthesized SiO 2 NPs were compared with the chemically synthesized SiO 2 NPs.
1. FTIR analysis for CS-SiO 2 NPs demonstrated two dominant peaks at roughly 1053 cm -1 and 786 cm -1 , which are associated with the asymmetric stretching vibration of the Si-O bond. While FTIR for GS-SiO 2 NPs revealed that the peak at 3468 cm -1 is related to the Si-OH, theat the peak at 3468 cm -1 is related to the Si-OH, which normally characteristics of the OH group match the accessible phytochemical in Rhus coriaria.
2. According to the XRD measurements, both GS and CS SiO 2 NPs had the same peaks at 2θ = 23˚, which confirmed the JCDPS reference. Although the intensity of the GS-SiO 2 NPs was higher than the CS-SiO 2 NPs, this was due to phytochemical presence in green synthesis.
3. The morphological studies revealed that the CS-SiO 2 NPs possess monodisperse NPs while the GS-SiO 2 NPs show polydisperse NPs. This is more likely due to the fact that in the green synthesis method, different phytochemicals are participating in reducing capping and stabilizing the process. Besides, the FESEM images revealed that CS and GS SiO 2 NPs are spherical in shape with a minimum degree of agglomeration, and the NP sizes were between 45~60 nm and 10~15 nm, respectively.
4. The effect of this agglomeration on the zeta potential analysis was not shown. In other words, although the particles are agglomerated, GS-SiO 2 NPs provide good NPs stability.
5. The DSC analysis of CS-SiO 2 NPs displays that there is one endothermic peak, centered at 115˚C and one exothermic peak, centered at 537˚C. Whereas the DSC analysis of GS-SiO2 NPs shows one endothermic peak, centered at 190˚C and two exothermic peaks, centered at 55˚C and 522˚C, respectively.
6. The TGA analysis shows that the GS-SiO 2 NPs are more thermally stable than the CS-SiO 2 NPs. It loses only 7% of its weight at~400˚C. The CS-SiO 2 NPs lose 90% of their weight at this temperature. The overall results revealed that the green synthesis of SiO 2 nanoparticles was better than chemically synthesized SiO 2 in terms of particle stability and thermal properties.